The present invention relates to a method for producing porous, spherical particles in the non-nanoscalar range of polymeric material.
Spherical particles are used, for example, on a large scale as catalysts, catalyst supports, adsorbents, drying agents or ion exchangers.
For most of the applications named, particles are required with a uniform spherical shape and a narrow spectrum of particle sizes, in order to make a more uniform packing and a high packing density, for example, in a solid bed reactor, possible. If the particles are used in a moving bed reactor, they are expected to have a higher abrasion resistance.
If the particles are to be used as catalysts or catalyst supports, they must have, in addition to a narrow particle size spectrum, a certain surface area and a specific pore volume.
The spherical particles composed of inorganic oxides are obtained, for example, by the generally known sol-gel method, in which, to begin with, a sol is produced in the form of a solution, suspension or dispersion. Subsequently, the sol is converted into droplet particles, which are thereupon caused to gel. During this process, the droplets assume a spherical conformation and solidify in a suitable gaseous or liquid medium. After a wet process, the product is dried and tempered. The finished product consists of spherical, porous, oxide particles.
On the other hand, polymer dispersions are tiny polymer globules, which are dispersed uniformly in a liquid medium. Approximately, 30% of all synthetic resins based on styrene, butadiene, acrylic acid derivatives or vinyl esters are produced and processed at the present time in the form of dispersions.
For emulsion polymerization, monomers, to begin with, are dissolved in water or emulsified in the form of very fine droplets with addition of surfactants. After the polymerization reaction is started, during which monomers join together to form long-chain molecules, the droplets grow at the expense of the dissolved monomer and finally cure with cross linking into small spherical, non-porous particles, 50 to 1,000 nm in diameter (depending on the surfactant content).
It is furthermore known that porous, non-nanoscalar polymer beads can be produced by suspension polymerization in the presence of inert components, which can later on be removed from the polymer.
The polymerization of the monomers is carried out here under such conditions, that the inert components are finely dispersed in the polymer particles formed during the polymerization.
In a subsequent step of the process, the inert components are removed by evaporation, extraction or some other way, which maintains the desired porosity in the polymer particles.
This method is suitable, however, only for polymers, the chains of which form a rigid, coherent lattice structure, such as cross-linked aromatic polymers (German patent 3,202,479).
Moreover, porous particles, composed of polyvinyl chloride, can be produced by atomizing polymer solutions in a heated stream of gas and, at the same time, evaporating the solvent. The choice of solvent is important here, since any molecular weight change is to be avoided and, in any case, no solvent residues may remain in the polymer.